I. Field of the Invention
The present invention relates to cellular telephone communications and cradle systems for cellular handsets that allow a customer to use the handset in a hands-free mode such as when driving a motor vehicle. More specifically, the present invention relates to an apparatus and process for dynamically controlling the power output of a booster amplifier that can be used in combination with virtually any such cradle.
II. Brief Description of the Prior Art
Over the past decade, use of cellular telephones has become widespread. The use of a cellular telephone handset while driving creates certain risks. First, such use can distract the driver. Second, one hand must be used to hold the phone to the ear and mouth, leaving only one hand to operate the motor vehicle. Such use presents sufficient risks that it has been outlawed in several jurisdictions.
Recognizing these risks, various manufacturers have begun to provide accessories that permit hands-free use of a cellular telephone handset while driving. Generally speaking, the solutions offered by these manufacturers take one of two forms. The first is a headset including an earpiece and a microphone that the driver can plug into the handset and wear while talking. Use of the headset allows the driver to operate the vehicle with both hands. The second is a cradle that includes a microphone and speaker into which the handset is placed. The microphone and speaker again allow the driver to carry on a conversation and use both hands to drive.
Cradle systems offer various advantages over headsets. First, a real issue with cellular telephones is the need to conserve the draw of electricity from the telephone's battery. Many cradles not only provide a separate power source, but also have the ability to recharge the battery of the handset.
Second, cradle systems connect to external antennas to increase range and signal quality. Further improvements can be made by amplifying the signals from the phone using a booster amplifier.
Most cellular telephones conserve battery power by reducing the power of the handset's transmitter to the lowest level that will still provide active effective communication. This is achieved by modulating the power output of the handset. More specifically, the base station controlling the cell in which the handset is located measures the strength of the incoming signal it receives from the handset. The base station then issues an instruction to the handset and the power output is adjusted to the lowest level adequate to maintain communications. Such operation does reduce consumption of battery power and also reduce interference between various handsets in the cell. However, it does exacerbate the chance of “dropped calls”. Dropped calls are annoying to the user of the handset and expensive for the cellular telephone network operator. Dropped calls occur when the signal is lost between the handset and the base station. This problem is particularly acute in cities where building can block the signal path and power adjustments are not made quickly enough. This same problem can occur when the user of the handset is traveling at high speeds through rolling countryside.
The prior art includes efforts to ameliorate this problem by boosting RF power. Four methods of boosting power when using a cradle have been described in the prior art. The four prior art cradle based, power-boosting techniques will be discussed in the context of the AMPS cellular telephone network used in North America. In the AMPS system there are three classes of cellular telephones. Class I telephones operate at a maximum power of 6 dBW. Class II telephones operate at a maximum power of 2 dBW. Class III telephones operate at a maximum of −2 dBW. Most handsets operate as Class III telephones to conserve battery power.
The first prior art technique to boost power of the Class III telephones is to provide the cradle with a second RF power amplifier. When the handset is placed in the cradle, the cradle's amplifier is activated so that the handset and the cradle work together as a Class I device. The amplifier in the cradle boosts the maximum RF power output to about 3.0 watts and thus provides a 7 dB advantage over a 0.6 watt handset. Such a cradle arrangement is shown in U.S. Pat. No. 5,457,814 to Markku Myrskog et al dated Oct. 10, 1995. A real problem exists, however, with the power boost system discussed in the Myrskog et al patent. It does not have universal application and a specific handset must be used with the cradle. It does not disclose a universal boost system.
The second prior art technique to boost power involves providing the handset with a second RF power amplifier. The handset senses when it is placed in the cradle and activates the second amplifier. This changes the handset from a Class III device operating at 0.6 watts to a Class I device operating at 3.0 watts. This technique is discussed in U.S. Pat. No. 4,636,741 to James E. Mitzlaff dated Jan. 13, 1987. Again, a special handset is required and the cradle cannot be used to boost the power of any handset.
The third prior art technique is, perhaps, the simplest and least expensive of the four. When the handset is placed in the cradle, the handset increases its power to the maximum permitted for Class III operation. At the same time, the power control circuitry is disabled so the power output is not modulated in response to control signals issued by the base station to the handset. This can lead to significant interference with other handsets.
A fourth technique is disclosed in U.S. Pat. No. 6,029,074 to David R. Irvin dated Feb. 22, 2000. This patent discloses a handset that is set up to operate as a Class II telephone. It has control logic that allows it to modulate its power output in response to power attenuation signals it receives from the base station of the cellular systems. The control logic is also designed so that when the handset is not in the cradle, the power output cannot exceed a predetermined maximum value even if the attenuation signal received from the base station calls for a higher output. Thus, the handset acts very much like a Class III telephone when not in the cradle with the power output capped at −2 dBW and like a Class II telephone when in the cradle with a maximum power output of 2 dBW or about 1.6 W.
Those working in the field understand that generating a signal with higher power will reduce the number of dropped calls. Simply boosting the power, however, creates other problems such as interference with other cellular telephones and interference with normal cellular network operations. While telephones such as those disclosed in U.S. Pat. No. 6,029,074 would address some of these problems to a limited degree, such solutions still limit the dynamic range of the handset. Interference with other handsets and normal network operations remain a possibility. Other problems associated with use of a cradle are simply not addressed. These include the inherent loss of RF power between the handset's antenna and the cradle's coupling device and the loss in signal strength due to the length of cable from the cradle to the external antenna. Also, the AMPS system is not the only type of network used. A properly designed cellular booster amplifier is needed for CDMA, TDMA, PCS, GSM and iDEN network applications as well. The booster amplifier must be designed to work with analog, spread spectrum and other digital networks. There is also a need to provide a system that can accommodate many different kinds of handsets, even those that do not have the control logic of the type discussed above or internal amplifiers that permit the handset to operate as a Class I or Class II device.